Semiconductor devices are manufactured by depositing many different types of material layers over a semiconductor workpiece or wafer, and patterning the various material layers using lithography. The material layers typically comprise thin films of conductive, semiconductive and insulating materials that are patterned to form integrated circuits.
One type of semiconductor lithography involves placing a patterned mask between a semiconductor workpiece and using an energy source to expose portions of a resist deposited on the workpiece, transferring the mask pattern to the resist. The resist is then developed, and the resist is used as a mask while exposed regions of a material on the workpiece are etched away.
As semiconductor devices are scaled down in size, lithography becomes more difficult, because light can function in unexpected and unpredictable ways when directed around small features. Several phenomenon of light can prevent the exact duplication of a mask pattern onto a wafer, such as diffraction or interference, as examples.
One problem that occurs in semiconductor lithography is referred to in the art as flare. Flare in optical systems is caused by the scattering of light, typically caused by a lens or other component of the optical system. If a lens is imperfect, light bounces off in different directions and is not periodic, rather than passing through the lens in a straight line, resulting in flare. Flare may be caused by non-uniformity in a lens material, for example. In microlithography, flare decreases manufacturing process windows by exposing resist in unintended areas, causing line shortening, which is an effect where features are made shorter than intended, and feature erosion, which is a thinning of the feature height as it transferred into the resist. Flare decreases the depth of focus (DOF), thus decreasing the process latitude. Consequently, it is critical to measure and minimize flare in order to increase process windows in semiconductor lithography.
One method used to quantify flare is referred to in the art as a Kirk test, in which the ratio of doses required to expose and develop a masked and unmasked area is computed. This method requires optical inspection, usually manual, to detect the dose at which a feature of known mask size is cleared. Other techniques rely on a scanning electron microscope (SEM) to determine the change in line-width in regions in proximity to variable degrees of opening in a chrome mask.
A disadvantage of current methods of measuring flare in lithography systems is that the methods rely on manual optical inspection or SEM measurement. These methods are difficult to automate and are time-consuming, and involve a degree of subjectivity.
Thus, improved methods of measuring flare in semiconductor lithography systems are needed in the art.